1. Field and of the Invention
The present invention relates to copying or recording devices and in particular to a new and useful magnetic brush development apparatus for use in electrophotographic copying apparatus or electrostatic recording apparatus.
2. Description of the Prior Art
The magnetic brush development apparatus is an apparatus for attracting a developer containing magnetic powder therein to a non-magnetic support member in which magnets are disposed, and for bringing the developer into contact with a latent electrostatic image bearing member at an image development section in order to develop the latent electrostatic image. The name magnetic brush comes from the fact that the developer becomes like a brush on the support due to the magnetic force of the magnets disposed in the non-magnetic support member.
As the developers that can be employed in the magnetic brush development apparatus, there are a two-component type developer comprising non-magnetic toner and magnetic carrier, and a one-component type developer consisting of magnetic toner. The one-component type developer can be classified into an electrically conductive toner and an electrically insulating toner.
Image development is performed by the toner charged to the opposite polarity to that of a latent electrostatic image being electrostatically attracted to the latent electrostatic image. In case of the two-component type developer, the particle size of toner is smaller than that of carrier particles and the toner is triboelectrically charged so that the toner clings to the carrier and a magnetic brush is formed. In the one-component type developer, the electrically conductive toner is charged by injection of charges or by electrostatic induction, while the electrically insulating toner is triboelectrically charged by some member of a developer container with which the toner contacts or during the transporation of the toner.
In case of the two-component type developer, the toner is securely charged, but some means for maintaining the mixing ratio of the toner and the carrier, or the toner concentration, is necessary in order to obtain a developed image of a predetermined image density. In contrast to this, in case of the one-component type developer, it is unnecessary to control the toner concentration, and it is simple to handle the toner although the charging of the toner is not always sufficient.
A sleeve-shaped or cylindrical member and an endless belt-shaped member are known as the non-magnetic supporting members for forming a magnetic brush thereon by attracting the developer thereto. A plurality of magnets arranged in a radial manner and a single rod magnet having magnetic poles on the peripheral surface thereof are known as the magnets to be disposed in the non-magnetic support member. The photoconductor for use in electrophotographic copying apparatus and the dielectric member for use in electrostatic recording apparatus are known as the latent image bearing members. The shapes of the latent image bearing members are drum-like, endless-belt-like, plate-like and sheet-like.
Either, or both of, the non-magnetic support member for holding the magnetic brush thereon and the magnets disposed inside the non-magnetic support member, are moved relative to each other, so that the magnetic brush formed on the non-magnetic support member is moved on the non-magnetic supporting member. The latent image bearing member is also moved at a predetermined speed and the magnetic brush on the non-magnetic supporting member comes in contact with the surface of the latent electrostatic image bearing member at a predetermined position where the non-magnetic support member and the latent electrostatic image bearing member come closest to each other, namely at a development section, so that the latent electrostatic image on the latent image bearing member is developed continuously.
The magnetic brushes on the non-magnetic support member are formed along the lines of magnetic force distributed on the non-magnetic supporting member 1 among the magnets 2, 3, 4 and 5 which are disposed in the non-magnetic support member as shown in FIG. 1. In FIG. 1, the distribution of the magnetic lines of force among only the magnetic 2, 3 and 4 are shown. Each line of magnetic force starts from the magnetic pole N and returns to the magnetic pole S, and the magnetic field is strongest at each magnetic pole. The magnetic brush becomes highest at each magnetic pole and lowest inbetween each pole. The magnetic brush stands out at each magnetic pole as shown by reference number 6 in FIG. 2. Therefore, normally, the magnets are arranged so that each magnetic pole of the magnets is located in the development section and development is performed by the highest portion of the magnetic brush.
Generally, the image density of developed image depends upon development time. The development time here means a period of time in which developer is in contact with a latent electrostatic image bearing member. Therefore, in the magnetic brush development apparatus, the development time is a period of time in which the magnetic brush is in contact with a latent electrostatic image bearing member at the development position.
As mentioned previously, since the latent image bearing member is moved at a predetermined speed, the period of time in which the magnetic brush is in contact with the latent image bearing member is related to the width w of the magnetic brush in contact with the latent image bearing member. Therefore, the development efficiency, namely the image density of developed image per unit time can be increased by broadening the contact width w.
As one of the conventional techniques for making the contact width w great, an apparatus is known in which plural non-magnetic support members for supporting the magnetic brush thereon are disposed in close proximity to the surface of a latent electrostatic image bearing member, whereby the contact width w can be substantially increased. However, this apparatus has some shortcomings that the apparatus is oversized and expensive. In order to eliminate such shortcomings, it is necessary to increase the contact width w by a single non-magnetic supporting member.
In the magnetic brush development apparatus as shown in FIG. 2, the contact width w of the magnetic brush is related to a gap d in the development section between a latent electrostatic bearing member 7 and a non-magnetic support member 1, since the magnetic brush stands out at each magnetic pole. The contact width w is greater in the bottom portion of the magnetic brush than in the top portion of the magnetic brush. Therefore, the smaller the gap d, the greater the contact width w.
However, there is a limit in reducing the gap d, since the smaller the gap d, the greater pressure the toner receives at the gap d, so that blocking of the toner occurs by the toner being solidified under the pressure. When blocking of the toner occurs, the solidified toner scratches the latent electrostatic image or the surface of the latent image bearing member. Therefore, in order to increase the development efficiency, it is important to reduce the gap d to the extent that blocking of the toner does not occur.
Since the gap d is related to the contact width w and accordingly to the development time, images with an uneven image density are formed when the gap d changes during development. In the conventional magnetic brush development apparatus, a spacer roller is disposed between a non-magnetic sleeve and a photoconductor in order to maintain a minimum gap d.
However, as mentioned previously, since there is a limit in increasing the development efficiency by reducing the gap d, it is necessary to increase the contact width w by some other method for raising the development efficiency. Furthermore, it is known that the image density is not varied when the contact width w is large enough even if the gap d is changed to some extent. Therefore, it is more advantageous to increase the contact width w by some method.
As another method of increasing the contact width w, there is proposed a method of increasing the width of the magnet in the development section. However, the larger the magnet, the greater the magnetic flux density and the stronger the magnetic brush, which may cause a risk of disturbing the latent image on the photoconductor when the surface of the photoconductor is brushed by the strong magnetic brush.
As a further method of increasing the contact width w, there is known a method of disposing two magnets 8 and 9 with s space therebetween and with their magnetic poles arranged in the same direction in the development section as shown in FIG. 3. In this method, a magnetic field is formed so as to have a peak of magnetic field intensity right above the two magnets 8 and 9, so that the contact width w of the magnetic brush 6 with the photoconductor 7 is increased in comparison with the conventional magnetic brush development apparatus as shown in FIG. 4. However, this method has the following shortcomings in comparison with the above-mentioned conventional methods. Namely, more magnets are necessary, and the assembling of the apparatus is more difficult. And since the magnetic fields of the two magnets are directed oppositely at locations the two magnets, the magnetic toner or the magnetic carrier existing on the portion above the space between the two magnets is magnetized in the polarity opposite to that of the magnetic toner or the magnetic carrier in the other portion, so that the chainlike arrangement of the toner or the carrier is interrupted by the two magnets. The oppositely directed magnetic fields of the two magnets 8 and 9 are illustrated in FIG. 5, in which the lines of magnetic force starting from the N pole are directed to the N pole in the respective magnets 8 and 9, so that the lines of magnetic force starting from the respective N poles are oppositely directed.
In the magnetic brush development process of a magnetic brush development apparatus, the development force can be represented by the following formula: EQU F=Fc-F.sub.M
where F represents the development force, and Fc represents the electrostatic attraction of a photoconductor for attracting the developer thereto, and F.sub.M represents the magnetic attraction for attracting the developer magnetically in the magnetic brush development apparatus.
From the above formula, it can be seen that the magnetic attraction F.sub.M serves as a negative bias with respect to the development force in the magnetic brush development apparatus. Referring to FIG. 14, there is shown a development characteristic of the magnetic brush development by employing the magnetic attraction F.sub.M as a parameter, with the amount of toner deposition M as ordinate and surface charge Q of a photoconductor as abscissa. The solid line indicates a development characteristic when the magnetic attraction F.sub.M is comparatively small, while the broken line indicates a development characteristic when the magnetic attraction F.sub.M is comparatively great. In either case, the development time is set constant. As can be seen from FIG. 14, when the magnetic attraction F.sub.M is small, the toner deposition begins to be saturated even if the surface charge Q is comparatively small. Accordingly, uneven development hardly occurs. Furthermore, when the magnetic attraction F.sub.M is set small, the amount of toner deposition M during a predetermined development time becomes greater than that in case the magnetic attraction F.sub.M is set great and accordingly, the development time can be shortened in comparison with that in case of a freat magnetic attraction F.sub.M when an equal amount of toner deposition is required. However, when the magnetic attraction F.sub.M is set small, background appears in the copy and sharpness of image is lowered, so that setting the magnetic attraction F.sub.M at a low level has an adverse effect on the image quality.